Frequently, defenses against predators incur trade-offs between benefits and costs. Therefore an accurate estimation of individual predation risk is important for many prey organisms. Predation risk is usually assumed to be correlated to predator density. On the other hand, individual predation risk should be inversely correlated to prey density. Inducible morphological defense systems are perfect for testing these assumptions because the predation risk related trait expression is directly measurable.
We used three cladoceran, one rotifer and one ciliate system with chemically induced morphological defenses to test the hypothesis that defense expression should be both predator density and prey density dependent. Indeed we found in all systems an increase in the defensive traits with increased kairomone concentrations. In accordance with the hypothesis the defense expression at a given predator density was reduced at higher prey densities. Our results indicate that the ability to accurately estimate individual predation risk evolved in diverse systems.